During the operation of a turbine power plant, there are various conditions which may occur necessitating an immediate shutting down or tripping of the turbine. For example, a loss of electrical load may create a dangerous overspeed condition; low bearing oil pressure may cause excessive wearing and serious malfunction of the turbine bearings; excessive wearing of the thrust bearing results in axial misalignment of the rotating blades resulting in serious internal turbine damage; insufficient condenser vacuum may cause overheating at the last row of turbine blading; or other contingencies may occur where it is necessary to shut down or trip the turbine rapidly to prevent an unsafe operating condition or damage to the turbine power plant.
A failure or delay in shutting off the steam to the turbine in the event of any of the above contingencies may cause extensive damage to various portions of the plant, necessitating expensive repairs and prolonged shutdown. Thus, it is necessary that such a system react quickly to specific contingencies. In a typical steam turbine power plant, hydraulic fluid is pumped at high pressure to a plurality of hydraulically operated valves for controlling steam flow. These valves are designed to open on an increase of oil pressure, and to close on a decrease in oil pressure. Governor valves control the steam flow to the high pressure turbine and interceptor valves control the flow of steam to the intermediate and low pressure turbine stages. Throttle valves which control the flow of steam to the steam chest upstream of the governor valves and reheat stop valves, which control the flow of steam from the reheater section of the steam generator to the intermediate and low pressure turbine stages upstream of the interceptor valves, are provided primarily for protective control of the turbine. The throttle valves are also used for turbine startup. Thus, when tripping the turbine, the throttle valves, governor valves, reheat stop valves, and the interceptor valves are rapidly closed. This is accomplished by releasing the hydraulic fluid pressure to all of the valves simultaneously in response to the detection of any one of several operational contingencies by remote means under control of the operator.
Typically, turbine tripping systems have relied on mechanical hydraulic automatic stopping mechanisms referred to as "autostop systems," to maintain under pressure, the valve control oil for the steam inlet valves. The operation of such systems is described in U.S. Pat. No. 3,931,714 issued Jan. 13, 1976 to which reference is made for a more detailed description thereof. However, as will become apparent herein, as therein, continued reliance on at least an aspect of the operation of such systems is manifestly meritorious in the event that a fail-safe mode of operation is dictated by an unforeseen fault status in the operation of the electrohydraulic trip system disclosed in the referenced U.S. patent.
As a general proposition, in turbine power plants where the controls are automated or controlled from a central office, it is desirable to maintain the reliability and rapid response of the hydraulic system and to eliminate the relatively slow operation, difficulty in adjustment, and limited range of response of the mechanical autostop assembly with its accompanying linkage, except to the extent that retention of portions of the latter provides failsafe operation of the former. Of paramount importance in maintaining uninterrupted operation of the turbine under safe operating conditions while insuring failsafe tripping of the turbine by mechanical means in the event of an unforeseen contingency is the ability to test, on line, the mechanical overspeed trip channel, an autostop system retained in systems more recently developed.